1. Field of the Invention
The present invention relates to continuous seam welding.
2. Description of the Prior Art
It has been previously proposed to weld an elongate seam by means of a laser beam and in particular to weld the side seam of a tubular can body made of sheet metal such as steel, black plate or steels coated with a metal such as tin, chromium, nickel or zinc, or steels coated with layers of chromium and chromium oxide. The steel or other sheet material to be welded may have an organic coating.
In order to meet the commercial demand for welded cans it is desirable to weld can bodies at a rate in excess of 45 m/min. W M Steen and J Mazumder reported in the Welding Journal, June 1981, in an article entitled "The Laser Welding of Steels used in Can Making" that the welding of thin gauge steel (either coated or uncoated) was a possibility at speeds up to 7 or 8 m/min.
U.S. Pat. No. 4,315,132 (Saurin) describes a laser welding process which could weld cylinders at speeds up to 22 m/min but this process is not adaptable for use in the high speed welding of can bodies because high quality continuous wave (CW) laser welds cannot be consistently obtained at speeds above about 22 m/min regardless of laser power.
European Patent Application No. 0 143 450 (SWS Incorporated/Sharp) describes a method and apparatus for pulsed high energy density welding. This patent specification teaches that if a pulsed laser is used so that a series of overlapping pulses are laid down one after another then, because of the slight pause between consecutive pulses of the beam, the melt pool has time to stabilise. Welding can then proceed without the instability in the melt pool that occurs with a continuous power laser at speeds up to 40 m/min or more. This patent also gives an indication of the problems which beset the high speed welding of thin sheet metal and it indicates that the answer to high speed welding is not merely an increase in the power of the laser (this point is also made by Steen). Excessive power in the laser beam simply creates an unstable melt which may become a permanent hole if part of the melt is lost.
The problems which need to be alleviated are:
(a) the Sharp system only works at speeds up to 40 m/min because at greater speeds the melt becomes unstable and the weld is prone to such surface irregularities and undercutting as would be unacceptable in can making; PA0 (b) the weld produced by a laser beam is narrow so any deviation of the butt joint, between the parts to be welding can result in the beam missing the joint completely or striking at an out of focus position: PA0 (c) at high power levels there is a finite limit to the speed at which a laser can be pulsed: above this limited speed, a continuous laser beam may be used but problems (a) and (b) remain; and PA0 (d) The intense localised energy of the laser beam and conductivity of the sheet metal to be joined give rise to rapid heating and cooling of the melt and risk of martensitic transformation.
Experience using such a welding process as Sharp's shows that there is a great difficulty in controlling the mechanical handling of the material to be welded at such high speeds. In addition at speeds in excess of about 45 m/min instability once again occurs in the melt pool and welding becomes impracticable.
In a paper entitled "The use of laser beam spinning to improve fit up and beam alignment tolerances when laser welding butt joints in sheet steel" by C J Dawes, published by the Welding Institute as Report 269; 1985 various methods to overcome some of these problems are described. Discussing the welding of metals, thicker than those used in the can industry, at speeds much lower than appropriate to can making three methods of manipulating the laser beam are described. In one method a spinning laser beam was used which if scaled up to the requirements of the can making industry would be required to spin at speeds presently considered impracticable for can making. In another method the laser beam was directed to follow an oscillating path spanning the butt joint but this required the beam to follow a long path length shown in FIG. 1 at D so slowing down the welding process. In a further method the beam was defocussed to a broader zone width but lesser intensity.
The Sharp patent showed that a way to increase speed was to allow the metal time to dissipate energy between welding pulses. However this has its limits in that if welding is fast enough there is insufficient time to cool down between welding pulses. In other words the practical effect is that there seems to be a limit to the amount of energy that can be pumped into the pieces to be welded over any particular time interval. We have discovered that this limit may be overcome by arranging for a series of non-overlapping weld pulses to be applied so that each weld pool has time to cool and possibly freeze before an overlapping pulse from a further, out-of-phase series, is applied. The longer the time lag between the first and subsequent adjacent weld pools the better. However, the time lag should not be too long in order to take advantage of the heat already supplied. By this means it should be possible to reach speeds of 100 m/min or more. The process does not have to be restricted to only two series of pulses, three or more could be used to fill the space between the welds formed by the first beam pulses.